The invention herein described relates generally to thermal neutron detectors and more particularly to an indirect thermal neutron detector including a scintillation detector with background gamma ray shielding.
Many nuclear measurements are made in the petroleum industry. Such measurements have been made by scintillation detectors using thallium-activated sodium iodide crystals that are very effective in detecting gamma rays. The crystals have been enclosed in metal tubes or casings to form a crystal package. The crystal package has an optical window at one end of the casing which permits radiation induced scintillation light to pass out of the crystal package for measurement by a light sensing device (photosensor) such as a photomultiplier tube coupled to the crystal package. The photomultiplier tube converts the light photons into electrical pulses that are shaped and digitized by associated electronics. Pulses that exceed a threshold level are registered as counts that may be transmitted up hole to analyzing equipment or stored locally.
Other detectors are used for detecting neutrons at thermal energies. Existing neutron detectors suffer from a variety of drawbacks which limit their application in certain environments such as in a well bore where it is subjected to high temperatures and high vibrations when measuring while drilling. Examples of neutron detectors known in the art include counters or ionization chambers filled with boron-containing gases (e.g., boron fluoride enriched with boron-10) in which neutrons are detected by the production of charged particles when neutrons react with boron-10. The charged particles are accelerated by the electric field toward a cathode or anode where they are collected to produce a pulsed voltage signal. The electric field is developed by a large voltage potential between a long thin wire and the chamber casing. A problem with this type of detector is electric field fluctuation which arises from the dynamics of the thin wire. The fluctuation produces a signal that is difficult to disassociate from the neutron induced signal.
A related type of neutron detector is a boron-lined counter wherein boron-10 is coated on the wall of the counter which may be filled with a gas other than boron fluoride. In this case the reactions take place in the thin coating close to the wall, with only one of the two charged particles having a chance of entering the interior volume of the counter while the other stops in the wall. This type of counter also suffers from the problem of electric field fluctuation when subjected to vibration.
Other neutron detectors use boron-doped plastic scintillators. However, these detectors cannot withstand the high temperatures encountered in down hole applications in the petroleum industry. Other scintillating materials can also be used, such as lithium-6, but such materials not only detect background gammas but also alpha particles and tritons associated with the lithium-6 neutron interaction.
The present invention solves problems that were previously unrecognized or thought to be insoluble, by using approaches that are contrary to conventional teachings. The invention satisfies a long felt need for a simple thermal neutron detector and particularly one that can be used in high temperature and/or high vibration environments. Still more particularly, the present invention enables the use of common inorganic scintillation crystals, particularly thallium activated sodium iodide crystals, for detecting thermal neutrons.
In accordance with the invention a thermal neutron detector comprises an inorganic scintillation crystal covered by an inner layer including a thermal neutron radiation absorbing material and an outer layer including a gamma ray shielding material covering the inner layer.
In a preferred embodiment of the detector, the thermal neutron absorbing material includes boron-10 and the shielding layer is composed of lead or other material that blocks environmental gamma rays from interaction with the boron-containing layer. Thermal neutrons enter the detector and are absorbed by boron-10 in the inner layer covering the scintillation crystal. The boron then decays by emitting an alpha particle and leaving a lithium-7 atom at an excited level. The lithium-7 then decays by emitting a 480 keV gamma ray which is detected by the scintillation crystal. The outer layer functions to shield a large portion of background gamma rays as well as Compton scattered gamma rays from interaction with the boron-containing layer.
Further in accordance with the invention, the thermal neutron absorbing material is uniquely packaged between the outer shielding layer and the scintillation crystal so as to reduce the susceptibility of the detector to signal noise induced by vibration and to increase the ruggedness of the detector. This is effected by forming the thermal neutron absorbing inner layer from a mixture of the thermal neutron absorbing material and a resiliently compressible carrier material. A preferred carrier material is silicone or resiliently compressible compositions containing silicone. As is preferred, the mixture may be cast on the crystal to form a sleeve which functions to mechanically support the crystal inside a detector casing.
According to another aspect of the invention, a ruggedized detector for detecting radiation other than thermal neutrons includes a detector element that detects a radiation of interest and also thermal neutrons, and a resiliently compressible shield composed of a mixture of an elastomer and thermal neutron absorbing material.
The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description and annexed drawings setting forth in detail certain illustrative embodiments of the invention, these embodiments being indicative, however, of but a few of the various ways in which the principles of the invention may be employed.